Human Gene Therapy

Jennifer Anderson

Copyright 1999

Everybody carries about half a dozen defective genes. Many don't know this, unless someone they know is affected by a genetic disorder.(1) The genetics of many diseases are passed from one generation to the next by inheriting a single gene, such as Huntington's disease. Many other diseases and traits are influenced by a collection of genes.(4) About one in ten people has, or will develop, an inherited genetic disorder. Approximately 2,800 specific conditions are known to be caused by defects, or mutations, in just one gene. Most of us don't suffer any harmful effects from our defective genes because we carry two copies of nearly all genes. In most cases one normal gene is enough to avoid all the symptoms of disease. If the affected gene is recessive, and we inherit two copies of the gene, the disease will develop. If the affected gene is dominant, only one copy is enough produce the disease. There are also X-chromsome linked genetic diseases.(1)

A potential approach to the treatment of genetic disorders in humans is gene therapy. Gene therapy is the delivery of a correct version of a mutated gene to a cell, where its expression will produce the normal protein and restore normal cellular function. The mutated gene must be delivered to the nucleus of the cell.(2)

There are essentially two forms of gene therapy, somatic gene therapy and germline gene therapy. Somatic gene therapy involves the manipulation of gene expression in cells that will be corrective to the patient but not inherited by the next generation.(4) Germline gene therapy involves the replacement of defective genes in the germline cells, which contribute to the genetic heritage of the offspring. It has the potential to affect not only the individual being treated, but also his or her children. Germline therapy would change the genetic pool of the entire human species, and future generations would have to live with that change. It is not likely germline therapy will be tried on humans in the near future due to ethical problems and technical difficulties.(1)

In vivo gene transfer is the introduction of genes to cells at the site at which they are found in the body. Ex vivo gene transfer is the transfer of genes into viable cells that have been temporarily removed from the patient and are then returned following treatment.(6)

Foreign DNA can be injected into the cell, or its entry can be facilitated by various chemical or electronic ways, but these methods aren't very efficient. One requirement for gene therapy is that sufficient amounts of corrective DNA are delivered to enough cells to be therapeutically beneficial. An ideal gene delivery vehicle or vector would be able to enter a large number of cells and integrate its DNA into the host's chromosomes. Coincidentally, some kinds of viruses are perfectly adapted to do just that.(2)

Viral vectors can be split into different groups: retroviral vectors, lentiviral vectors, and adenoviral vectors. A few more are under investigation.(5) The principle of all these vectors is to remove the disease causing components of the virus and insert recombinant genes that will be therapeutic to the patient. The modified viruses cannot replicate in the patient, but do retain the ability to efficiently deliver genetic material.(4)

Retroviruses have a limitation because they are unable to infect non-dividing cells. This problem was overcome by the ex vivo gene transfer.(5) Richard Mulligan and his coworkers created a retrovirus capable of infecting cells and splicing a corrective gene into chromosomes. The problem with retroviruses is that scientists have no control over how many copies of the gene become integrated or where on the chromosome they insert. The vector's genetic load may be inserted into a different gene, disrupting its expression. It could also put a cell onto a path of cancerous growth if the gene integrates within the regulatory region of a gene responsible for controlling cellular proliferation. These must be considered, even though they are remote possibilities.(2)

Lentiviruses are a special type of retrovirus. The most notable is HIV. These viruses are able to infect non-dividing cells and can therefore be used for in vivo gene transfer. This system offers a targeted delivery and stable expression of the genetic material that is delivered.(5)

Adenoviruses can infect both dividing and non-dividing cells.(5) Like retroviruses, adenoviruses deliver their genetic load to the nucleus, but (except under rare circumstances) the genes do not integrate into the resident chromosomes. This relieves concern about random genetic integration, but it also means that the therapeutic gene is only temporarily active. The adenoviral vectors have to be repeatedly administered in order to maintain a steady therapeutic dose.(2) A disadvantage of adenoviral vectors is that the host is producing an immune response. This kills the infected cells and leads to the production of antibodies, preventing further infection by the same virus. But, nevertheless, it is useful in short term treatment, such as fighting off cancer cells.(5) Other drawbacks are it can only carry a small genetic load, it carries the risk of disrupting functioning genes by randomly inserting itself into the chromosomes, and it is somewhat difficult to manufacture these vectors in sufficiently high quantities.(2) Adenoviruses can infect a broad range of human cells. Brain tumors and mutations in the cystic-fibrosis transmembrane receptor gene have been treated with adenoviral vectors.(2)

Inserting the gene is only part of the problem. The vector must also contain a mechanism for activating the therapeutic gene, which is not automatic. Gene therapy must include a timing and regulatory device, or promoter. Promoters are difficult to place in a therapeutic vector because they are often very complex and sometimes quite large. In some cases therapeutic genes entered the cells as expected, but this was not always the case in the Mulligan experiments. Low levels of expression continue to disrupt gene therapy research.(2)

In addition to viral-based vectors, scientists are exploring non-viral delivery systems. One such system is delivery of drugs via liposomes. Liposomes are small vesicles artificially created from lipids that resemble those making up the membranes of mammalian cells. The liposomes can fuse with cell membranes and empty their contents, which can be drugs or corrective genes, inside the cell. Some of the DNA delivered by liposomes makes its way into the cell's nucleus.(2)

William French Anderson, Michael Blaise, and Ken Culver performed the first successful gene therapy on a human in 1990. They developed a plan for treating Adenosine deaminase (ADA) deficiency, a severe combined immune deficiency. ADA deficiency is a result of inheriting two copies of the defective ADA gene, therefore it is a recessive disease. Without at least one properly functioning gene, the result is an almost complete failure of the immune system and an early death. Using a genetically altered mouse retrovirus as a vector, a properly functioning ADA gene was spliced in to the RNA. The vector delivered its genetic load to the T-lymphocytes, as the protein shell of the retrovirus binds to the cell's receptors. During cell replication, when the cell is actually synthesizing DNA, the RNA from the retrovirus is converted into DNA and integrated into the DNA of the cell. The genetically altered cell now has a functioning ADA gene that produces ADA within the cell. Cultured T-lymphocytes are then reintroduced into the children. The first children to have the gene therapy ( in combination with shots of polyethylene glycol coated ADA) had extremely large increases in their immune functions.(3)

As I mentioned earlier, experiments on humans with brain tumors have begun. In these experiments, patients were chosen who had severe tumors and were viewed as terminal with weeks to live. Five of the eight patients studied showed tumor regression. Experimentation continues on brain tumors and several other kinds of cancer.(3)

On September 2, 1999, 36-year-old Donavon Decker from Huron, South Dakota became the first person to receive a gene therapy injection for muscular dystrophy. Decker has limb-girdle muscular dystrophy (LGMD). Dr. Jerry Mendell, chairman of the Department of Neurology at the Ohio State University Medical Center in Columbus, injected a muscle on the top of Decker's foot with genes for a muscle protein that is missing because of a genetic flaw. Decker received the genes for a sarcoglycan protein that belongs to a cluster present in healthy muscle cell membranes. This protein cluster is thought to protect the membrane during contraction and relaxation. The genes were injected using a highly modified, "adeno-associated" virus developed by MDA researchers led by Dr. James M. Wilson, director of the Institute of Human Gene Therapy at the University of Pennsylvania in Philadelphia. One of Decker's foot muscles received the therapeutic genes, while the same muscles on the other foot received a phony injection. Researchers will take biopsies to see whether the genes reached the cell nucleus, if there is any damage to the muscle cells, whether the needed sarcoglycan protein was produced, and if that and other proteins normally associated with it ended up in the right place in the cells. Mendell said eventually they'll be looking to see if gene therapy can stop the progression of muscular dystrophy all together.(7)

The last three examples have been successful so far, but this is not always the truth. On September 17, 1999, 18-year-old Jesse Gelsinger from Arizona died while participating in a gene therapy experiment at the University of Pennsylvania. Gelsinger suffered on and off from a serious genetic disorder call Ornithine transcarbamylase (OTC), which blocks the body's ability to break down ammonia that often leads to coma and death in childhood. This was the first experiment in which a virus was shot directly into the liver's blood supply. After getting the virus injection, which was meant to deliver a gene that would help him make the enzyme he lacked, his organs progressively failed over the next three days. His death was an unexpected event that has caused federal officials to halt such experiments while undergoing an investigation.(8)

Gene therapy has enjoyed few successes and many failures. But within these failures, lessons have been learned. Many now see it as having the potential to change the way medicine is practiced. Its uses are varied, publicity and public comprehension suggest that gene therapy will soon join the ranks of antibiotics, vaccinations, and organ transplants.(3)

Even the most successful clinical trial has fallen short of therapeutic effectiveness. Most of the trials have been highly publicized. Having failed to live up to the overrated expectations created by such publicity, these disappointing trial results have left a general impression that gene therapy cannot now or ever fulfill its initial promise of solving the problems associated with genetic disease.(2)

Gene therapy continues to offer a lot of hope for the future treatment of a variety of clinical conditions. The development of fitting, unusual gene transfer vectors will improve the performance and constancy of therapeutic gene expression in the many settings of gene therapy. In the context of tissue and organ transplantation, gene therapy is being put together to prevent the severe and habitual rejection of transplanted tissues by introducing either new genes that are important in preventing rejection or antisense nucleic acids to block the production of rejection-associated molecules. The delivery of genes by gene therapy vectors that encode for alloantigens might also be an effective way of developing immunological tolerance in the recipient, perhaps eliminating the requirement for potentially harmful whole-body immunosuppression.(6)

While gene therapy shows potential, opponents of biomedical reductionism will undoubtedly have several responses. The way a person experiences a disease involves many social and psychological implications, such as the emotional impact of a disease, the stigmatism attached, and the cost and employment implications. Clearly there is public excitement about the prospects of genetic medicine. At the same time, there are financial forces promoting these approaches, from private biotech firms, to government funding. The result of the increasing focus on reductionism measures is the neglect of the social factors which studies show are key determinants of the health of populations. Consideration of the main causes of death leave open the possibility that emphasis on gene therapies may not be the best allocation of resources. Gene therapies may be very promising, but an estimated fifty percent of all deaths in the U.S. in 1990 were preventable. The history of medicine, and a knowledge of the primary determinants of health, calls into question the biomedical reductionist assumptions of this project.(3)

Increases in public funds for research may be part of the answer to improving technology. It is the basic level of investigation that will, in the end, broaden understanding of gene therapy techniques and increase the probability of clinical success. What is clearly needed, is the development of molecular analysis and rigorous testing at the level of basic science, with an eye towards application in clinically appropriate targets.(2)

My opinion on gene therapy is that it may have good implications for future generations that are known to have detrimental genes. For example, if parents know of genes in their families, gene therapy can be used to prevent their children from genetic disease. This can be accomplished by germline gene therapy. The development of serious and unfavorable inherited genetic diseases could be prevented before birth and eliminated in the following generations. By using this technique, though, it raises many ethical and safety issues that must be evaluated completely before trial on humans.

The future of gene therapy right now to me looks promising, but there is a lot of work to be done. I don't think we should be trying these experiments on humans until all the problems have been worked out. If it is tried on humans, all angles should be looked at so that if something should go wrong, as in the Gelsinger case, it can be reversed to avoid multiple organ failure, or whatever complications arise.